Recently, we described new processes for making polyetherester resins from polyethers (see U.S. Pat. Nos. 5,319,006, 5,436,313, and 5,436,314, and 5,677,396). In each process, a polyether reacts with a cyclic anhydride, a dicarboxylic acid, or a diol diester in the presence of an "insertion" catalyst. The anhydride, dicarboxylic acid, or diol diester inserts randomly into carbon-oxygen bonds of the polyether to generate ester bonds in the resulting polyetherester resin. The polyetherester resin is then combined with a vinyl monomer, preferably styrene, and is cured to produce a polyetherester thermoset. Lewis acids, protic acids having a pKa less than about 0, and metal salts thereof are effective insertion catalysts. The insertion process provides a valuable and versatile way to make many unique polyetherester intermediates.
More recently (see U.S. Pat. No. 5,696,225), we extended the insertion technology by developing a process for making high-performance polyetherester resins. These high-performance resins are made by chain-extending a polyetherester resin (made by insertion) with a primary diol or a diepoxy compound. The high-performance resins give thermosets with improved high-temperature performance, better tensile and flex properties, and enhanced resistance to aqueous solutions compared with those made using the earlier polyetherester resins. The thermosets are produced by reacting the unsaturated resin with a vinyl monomer. Unfortunately, high-performance polyetherester resins made by chain extension have relatively high molecular weights and viscosities, which can make formulating with them challenging. Preferably, good water resistance could be achieved with a low-viscosity, low-molecular-weight resin. In U.S. Pat. No. 5,696,225, we also described a way of making thermosets by co-curing the unsaturated polyetherester resin in the presence of both a vinyl monomer and a diepoxy compound (see, in particular, Examples 24-30 of the '225 patent). Co-curing the polyetherester resin in the presence of both a vinyl monomer and a diepoxy compound resulting in exceptional improvements in water resistance, especially flex strength retention following a six-day water-boil test.
While the co-curing process described above does give water-resistant thermosets, it has some limitations. First, diepoxy compounds react rapidly with carboxylic acid groups but more slowly with hydroxyl groups. Consequently, the polyetherester resin usually needs to have a relatively high acid number (at least about 40 mg KOH/g) to give a desirable curing profile. Second, the curing reaction of carboxylic acid groups with the diepoxy compound, which usually occurs at elevated temperature, can be challenging to coordinate with the low-temperature free-radical curing reaction. Finally, some diepoxy compounds are expensive. Preferably, water-resistant polyetherester thermosets could be made without the need to co-cure with diepoxy compounds.
Other known co-cured thermoset systems include "hybrid resin" systems such as those originally developed by Amoco Corporation under the Xycon trademark (see, e.g., U.S. Pat. Nos. 5,153,261, 5,296,544, and 5,344,852). These systems typically react a hydroxyl-terminated unsaturated polyester, a diisocyanate, and a vinyl monomer in the presence of a free-radical initiator. Other active hydrogen compounds are usually included. The unsaturated polyester resins are made by conventional condensation polymerization techniques. Hybrid resin systems offer advantages of polyester and polyurethane technologies. However, some of these hybrid systems lack adequate tensile and flexural strength properties. In addition, some hybrid systems require a resin having a very low acid number (&lt;10 mg KOH/g). Preferably, good tensile and strength properties could be achieved with resins having higher acid numbers (e.g., 30 mg KOH/g or higher) while preserving the other advantages of hybrid resin systems.
Thermoset products with high thermal dimensional stability, commonly measured in terms of DTUL, are always in demand in the composites industry. Unfortunately, there are a limited number of ways to achieve high DTULs while maintaining adequate strength and flexibility.
In sum, the composites industry would benefit from new ways to make water-resistant thermosets. Preferably, good water resistance could be achieved with low-viscosity, low-molecular-weight resins instead of relatively high-molecular-weight, chain-extended resins. Valuable water-resistant thermosets would avoid the need to co-cure with diepoxy compounds. Moreover, the industry has a need for hybrid resin systems that have--in addition to good water resistance--improved flexibility and strength properties. Finally, there remains a need for plastics with good thermal dimensional stability (high DTUL) while maintaining good flexibility and strength.